1. Field of the Invention
This invention relates to molding methods, to rotomolding methods, to blow molding methods, and in one aspect to methods for imparting decorative and printed matter to molded parts which become an integral member of the molded part.
2. Description of Related Art
In certain prior art rotational molding methods, molded parts are formed within a mold cavity by charging the cavity with particulate molding resin, then heating the cavity to a molding temperature while rotating or tumbling the mold, usually in two or more axes, for a sufficient time to disperse the molten resin particles into a molten film which coats the interior walls of the cavity. Thereafter the mold is cooled until the rotomolded part has solidified, then it is opened and the part is removed from the mold. One disadvantage of this molding technique is that it is difficult and/or ineffective to apply decorative or printed matter to the molded part after it is molded. This is aggravated by use of conventional molding resins, e.g., polyolefins, etc., which form surfaces that are not readily receptive to coatings of paints, inks, decals, and the like. Some attempts have been made to chemically or physically treat the surfaces of rotomolded products, e.g., by flame treatment, to render the surfaces receptive to printing inks. These procedures are costly and the resulting patterns are sensitive to abrasion.
U.S. Pat. Nos. 4,252,762 and 4,519,972 proposed a system in which patterns are formed on a transfer film, then subsequently transferred from the transfer film to an inner surface of a mold. In this process pigments in the molten pattern migrate into the molding resin as it coalesces during the molding process. Although a pattern is formed as part of the molded product, this process has shortcomings: pattern sizes are limited due to mechanical restrictions in screen printing; long patterns must be pieced from several screen-printed sheets; color and pattern matching is difficult; the transfer of the pattern from the carrier onto the inner mold surface is difficult, costly, and time consuming; burnishing is required to transfer the pattern from the carrier film to the mold; this burnishing is not only troublesome but also may injure the mold surface, requiring frequent mold refinishing; errors in burnishing of the pattern may be costly; if a fragment of the pattern is not burnished properly, it will not transfer, leaving a hole in the transferred pattern; if the defect is not noticed, a defective molded part is produced; if it is noticed, the removal of the old pattern prior to replacing it with a new one is difficult and can result in a damaged mold requiring extensive rework; the transfer patterns are very thin and fragile and their adherence to the carrier film is, by design, precarious since they must release easily from the transfer carrier when transferred to the mold; cracking due to brittleness of the pattern is common; fractures of the pattern during application are common; shipping damage is common; weather temperature extremes can render the patterns useless; the transfer pattern melts at a lower temperature than does the particulate resin used to mold the part; it will not transfer from the transfer film to the mold wall properly if the mold is too hot; if the molded part is removed from the mold while the mold temperature is too high for pattern transfer, then a delay period of time must be introduced to provide a mold cooling period; such delay adds to the mold cycle time thereby increasing molding costs; the transfer pattern becomes molten during the early portion of the molding cycle, prior to the melting of the resin particles; the mold charge is typically in the form of pellets and if allowed to flow over the image too long, erodes the pattern away, resulting in a ruined molded part; in order to avoid this damage, the mold must be preheated before being agitated; this preheat stage adds further complication and cost to the mold cycle; the transfer pattern leaves a faint `ghost` image on the surface of the mold after the molded part is removed from the mold; if the next application of a pattern to the mold is not in exactly the same location, this ghost image of the previous application will appear on the new part in addition to the proper pattern; the part is thus cosmetically defective; careful placing of the pattern is thus required to avoid this defect; the extra time and danger associated with this careful placement adds to the cost of the end product part; a transfer pattern cannot be moved in its location on the mold wall, once burnishing is started; if an error in location is noted, the entire burnished area must be removed, the transfer pattern discarded, and a new one located properly and burnished into position; transfer pattern removal can easily cause serious damage to the polished surface of the mold wall. These factors can disrupt the molding schedule drastically, adding further to the cost of the molded end product part.
The mold cavity used in a typical blow molding process usually does not subject the graphic to as severe an environment as does a typical rotomolding process. In blow molding the mold is fixed in position during operation, as opposed to the rotomolding process in which a mold is rotated constantly through at least two axes throughout the mold cycle. Also, in blow molding a much narrower range of temperatures is used. It is customary in the blow molding industry to use vacuum to hold a graphic in place during a mold cycle. The mold is drilled through for the application of the vacuum. The graphic is placed into position on the inner face of the mold and the vacuum is applied to hold the graphic in place throughout the mold cycle. In blow molding, the heating is done on the tubular film suspended within the mold, as opposed to heating the mold in the case of rotomolding. The heated film is then blown against the mold while it is soft. The mold is kept cool enough to cool the film when the film contacts it, thus freezing the film in its new conformation. Rotation of a rotomolded mold in two axes makes it difficult to design and build a mechanism which allows the use of vacuum to hold a graphic in place during a molding cycle. In addition, the use of vacuum as an in-mold graphic holding device in rotomolding has other drawbacks: since the graphic approaches its molten state during the mold cycle, it tends to leak into vacuum inlet holes in the mold, resulting in a molded part being stuck to the mold at the completion of the molding cycle and in disfigurement of the molded part surface over the area of applied vacuum--an unsightly bump would appear on the surface of the molded part, corresponding with each vacuum inlet hole. Many adhesives cannot be used since one strong enough to hold the graphic in place during the rigors of the mold cycle would also inhibit release of the finished part when the mold cycle is completed. Other adhesives, especially those which contain volatile solvents, generate gas bubbles in the mold-graphic interface and contribute to weakened and ugly walls in a molded part.
Various areas of difficulty are encountered in the use of adhesive systems:
--The end location of the graphic on the surface of the finished part should be consistent from one molded part to the next;
--The placement of the graphic on the inner surface of the mold must also be consistent and the molder should be able to get the same end result with each graphic placement;
--The graphic should not come loose from the mold during the molding process, either as the result of the tumbling process, the heating and cooling cycle, or the abrasion of the particulate resin during the tumbling; and if the graphic curls or tumbles during a rotomolding process, the molded part will be cosmetically ruined;
--The system used for attaching the graphic to the inner surface of the mold should not allow any trapped air between the two;
--The adhesive used should cease its function at the end of the molding cycle so that the molded part can be easily removed from the mold; and
--Although the graphic should remain in the same location on the inner surface of the mold throughout a rotomolding cycle, it should nevertheless be allowed the freedom to grow and shrink along that surface as the mold temperature changes; since the graphic does not have the same coefficient of expansion with heat as the mold, it elongates more than the mold as the temperature rises, then shrinks more as the temperature falls. As the mold heats up, the graphic should be allowed to grow on the mold surface, otherwise the stress introduced by expansion will cause the graphic to buckle at right angles to the direction of expansion, leaving an unsightly and wall-weakening defect in the molded part wall.
Graphics which are mounted on a transfer sheet, and which are subsequently burnished from the transfer sheet onto a rotomolding mold surface, are limited to two dimensional displays and may only be applied on plane surfaces (i.e. curvature allowed only in one of the two dimensions). There is a need in the industry for in-mold graphics which can be fitted to more complex mold inner surfaces. An example of such a need is the tail-light assembly in a rotomolded toy automobile in which the graphic must conform to a shape which wraps around the rear corner of the automobile with curvature in at least two dimensions. Unlike prior art transfer graphics, certain film graphics described herein which are composed of only thermoplastic materials, may be further thermo-formed into a replica of the mold inner surface, using standard prior art industry thermo-forming practice, thus forming a mold fitting part when installed in the mold. Such an application of in-mold graphics has not been heretofore available.
The following U.S. Patents describe various molding methods and techniques: U.S. Pat. Nos. 3,079,644; 3,420,729; 3,492,391; 3,796,622; 3,953,564; 4,089,922; 4,183,883; 4,267,229; 4,318,683; 4,548,779; 4,668,450; 4,880,588; 4,895,690; 5,073,325; and 5,311,816.